The invention relates to a sleeve for flexographic printing, having a sleeve body made of an at least partially non-electroconductive material, which is mountable with its inside on an electroconductive king roll of a printing press designed as an air cylinder, and which in the assembly state exhibits at least one contact zone with the king roll.
When printing foils, paper sheets, paper pages, and other substrates, the use of so-called sleeves (working sleeves), which are mounted on king rolls of printing presses using an air cushion and which hold the printing block, made of rubber or other elastic synthetic photopolymer, for a subsequent printing process, has increasingly been gaining acceptance. The king rolls of the printing presses, which are almost without exception made of steel cylinders, are for this purpose provided with an internal air system leading to bore holes on the outer wall of the roll, which are impinged on with compressed air when the sleeves are axially pushed and pulled in order to slightly stretch the sleeve body of the sleeve. The sleeve bodies of the sleeves are in particular made of glass fiber reinforced plastics in order to be able to reversibly absorb these elongations. In sleeves for flexographic printing with wall thicknesses of more than 5 mm, at least one compressible flexible foam layer is additionally provided in order to ensure the stretching of the sleeve body during mounting or dismounting despite the considerable wall thickness. Sleeve variations are also known, in which the entire wall structure is made of a compressible material. After mounting, and with the compressed air switched off, the sleeve sits in a press-fit on the outer wall of the roll of the king roll and is rotationally fixed thereto.
In order to print on foils, paper sheets, or paper strips, solvent-containing colors are used, with a multi-roll system with ink duct and color ductor or other suitable inking apparatus indicated for printing being used in the flexographic printing press for each color. In modern flexographic printing presses, there is an increasing need for a great number of printing couples and, at the same time, for large-format king rolls or sleeves. Among experts, it was previously assumed that in flexographic printing, because of the small number of printing couples, the problem of electrostatic charging is negligible, and even in combustible solvents, such as alcohols, glycol or carboxylic acid ester used in flexographic printing, an ignition or deflagration of the solvents in the colors is not expected. A reason for this may have been the small number of printing couples previously existing in printing presses.
However, because of higher demands on work safety and the need for flexographic printing presses with a greater number of printing couples, the problem of electrostatic charging even in printing presses for flexographic printing can no longer be ignored. Electrostatic charging may occur in various areas of the printing press. For instance, they may occur between the substrate to be printed on, such as a foil, and the printing block, because of the lifting motion and because of excessive charging of the dye. If suitable protective measures are not met, as the number of printing couples increase, the electrostatic charges increase as the substrate to be printed on advances in the machine, so that dangerous discharges into the ink ducts or into the chamber color ductor may occur in the rear printing couples as seen from the direction of processing.
It is basically known to the expert that the scale of the charging depends on the surface finish and the conductibility of the substrate to be printed on, its speed of passage through the inking systems, the surface finish, the conductibility, as well as the modes of operation of the devices used in the inking system (e.g., color ductor, guiding elements, color chamber, etc.), as well as the ambience conditions in the printing press. Electrostatic charges may be limited by suitably adjusting to the ambience conditions, in particular keeping an atmospheric humidity of about 65% and/or air ionization in the working area.
The task of the present invention is to create a sleeve for flexographic printing that is made of an at least partially non-electroconductive material so that it may be mounted following the air-cushion principle, and at the same time, minimizes or avoids the problem of electrostatic charging.
This task is achieved with the present invention specified in the independent claims. Advantageous embodiments of the present invention are specified in the sub-claims. According to the present invention, the sleeves made of a (partially) non-conductive material are provided with an electroconductive structure through which the unavoidable electrostatic charges are directly diverted via the king roll, so that the charging of the individual sleeves or application rollers in the printing couple cannot reach a scale sufficient for a discharge, ignition, or deflagration of the solvents, and are also not passed on to subsequent printing couples. This is achieved in that the sleeve is provided with an electroconductive surface that is connected to the contact zone via at least one electroconductive element provided in the sleeve body for diverting electrostatic charges into the outer wall of the roll of the king roll. The electroconductive surface of the sleeve, and therefore, the sleeve for flexographic printing, is grounded via the element provided in the sleeve body and via the outer wall of the roll of the king roll. The sleeves according to the present invention are given an electroconductive structure that does not add to its weight. The modest weight desired by the user may also be achieved or preserved with the sleeves according to the present invention.
Within the scope of the revelation of the present invention, electroconductive substances or materials are, in particular, meant to be those whose specific resistance is less than 104 xcexa9m. However, since what matters in the invention is the diversion of the electrostatic charge, substances or materials that are electrostatic dissipative and whose specific resistance is between 104 xcexa9m and approximately 109 xcexa9m may also fall under electroconductive substances or materials. Consequently, such substances and materials whose specific resistance is greater than 104 xcexa9m or which are neither conductive nor have a dissipative ability are not conductive.
The electroconductive surface of the sleeve may be formed or applied in various ways. In an embodiment, the sleeve body may be provided on its outer surface with a conductive lacquer forming the surface of the sleeve. A varnish may be given the desired conductibility, for example, by mixing with conductive particles, such as soot, or with metal pigments, such as silver or copper or the like. Alternatively, the surface of the sleeve body may be made of a conductive synthetic layer, in particular a conductive polyurethane layer, which is already provided during the build-up of the sleeve or is applied only subsequently. The synthetic layer may, in particular, also be made of glass fiber reinforced plastics (GFRP) having a conductive matrix, or of a carbon fiber reinforced polymer (CFRP), which is conductive due to the carbon fibers. Alternatively, the sleeve body may be provided on its surface with a metallic coating forming the surface of the sleeve. The sleeve body may, for example, be vacuum-coated with a suitable metal. The application of the conductive lacquer as well as the application of a metallic coating makes it possible to retrofit already existing sleeves for flexographic printing and is supposed to subsequently make them conductive in a sleeve service.
The electrical conductivity of the sleeve may alternatively also be achieved by making the sleeve body out of a composite material with electroconductive inserts forming an interlacing of fasteners. In a preferred embodiment, the sleeve body may for this purpose be made of a metal fiber or carbon fiber reinforced polymer (CFRP), while ensuring during the manufacture of the sleeve that the inserts or fibers extend not just axially, but also radially from the contact surface up to the surface.
A sleeve completely made out of CFRP is mountable only in thin wall thicknesses of up to about 5 mm. For thicker wall thicknesses, the sleeve body is normally provided with at least one layer of non-conductive, compressible material in order to ensure the size of the sleeve for the mounting or dismounting. For such sleeves, it is provided according to the present invention for the layer made of non-conductive material to be penetrated by the element for diverting electrostatic charges. In the preferred embodiment of a sleeve with a non-conductive, cylindrical layer or interlayer, the element is provided in a radially extending recess in the sleeve body. The recess may be formed by a radial borehole, in particular. So that the diverting element for the electrostatic charging does not hinder the compressibility of the layer made of a non-conductive material, it is particularly favorable in this embodiment if the element may change its length in radial direction, and after the mounting, independently adjusts to the length necessary for the electrical connection. This may preferably be achieved by having the element exhibit a spring-loaded contact body. It is particularly advantageous if the contact body is arranged in such a way that it juts out beyond the inside of the sleeve body before the mounting. Using the spring, the contact body is therefore preloaded towards the inside of the sleeve, or pressed in the mounting state against the outer wall of the roll of the king roll, thereby ensuring the electroconductive connection between the conductive surface of the sleeve and the electroconductive king roll. In order to facilitate the assembly of the sleeve and prevent obstructions to the lifting motion at the element, it is advantageous if the contact body is formed by a ball or at least exhibits a spherical contact end.
In order to able to install the element in sleeves in a simple manner, it may exhibit an electroconductive housing, e.g., made of metal, which contains the spring and the contact body, and with its housing bottom, is connected to the conductive surface. As set forth at the start, already existing sleeves or sleeve bodies may subsequently be made conductive. For this purpose, a sleeve body made of non-conductive material, in particular, may subsequently be provided with at least one radial through-bore hole, into which an element is inserted for electrostatic diversion before the conductive surface is applied. The element may, in particular, be screwed or glued in place into the through-bore or the recess. It is furthermore favorable if the recess or through-bore is countersunk on the inside of the sleeve body so that the air cushion occurring between the inside of the sleeve body and the king roll during the mounting or dismounting of the sleeve may not result in a displacement or loosening of the element or its housing.
In order to ensure a uniform concentricity of the sleeve and/or to ensure a complete diversion of the occurring electrostatic charge in large-surface sleeves, several elements may be provided, in particular in a rotationally symmetrical manner, for diverting the electrostatic charge.
The contact zone need not inevitably lie on the inside of the sleeve. Most king rolls are provided with a centering overhang for the register adjustment, with the sleeve using the centering overhang to center itself with a centering recess during the mounting. In such king rolls, the contact zone may thus be a component of the centering recess, resulting in the further advantage that the compressibility of the interlayer is not affected. In order to ensure the contact of the contact zone with the centering overhang, spring-loaded contact elements may be provided in the centering recess.
Alternatively, the sleeve body, as an element for the electrostatic diversion, may exhibit at least one fiber made of a conductive material, in particular of metal.